How to calculate round port to slot port? Or horn options?

Hi all, I am building a small subwoofer for my car. I am thinking of using a Rockford Fosgate 8" P2D2-8 DVC 2ohm. They recommend a .64cuft box with a 8.8" x 2" port. I would like to make a slot port but don't know how to do the calculations to create the right dimensions.

Also, if a horn can be made off this drive and can keep the box relatively small, that might be even better.

I am replacing dual 6x9 "subwoofers" in my car. The magnets on the drives state 2ohms and 25watts. They really suck and don't produce much bass. I am trying to run this new sub off my stock amp.

Any help would be appreciated..

Yeah, that could be a problem.

I am limited to a 2ohm DVC design because i am sticking with the factory amp. Is there a design out there for a very efficient small subwoofer that I can use a 2ohm DVC drive with?